Scanning microscopy techniques, including near-field and confocal scanning microscopy, conventionally employ a single spatially localized detection or excitation element, sometimes known as the scanning probe [xe2x80x9cNear-field Optics: Theory, Instrumentation, and Applications,xe2x80x9d M. A. Paesler and P. J. Moyer, (Wiley-New York) (1996); xe2x80x9cConfocal Laser Scanning Microscopy,xe2x80x9d C. Sheppard, BIOS (Scientific-Oxford and Springer-New York) (1997).] The near-field scanning probe is typically a sub-wavelength aperture positioned in close proximity to a sample; in this way, sub-wavelength spatial resolution in the object-plane is obtained. An aperture smaller than a free space optical wavelength of an optical beam used in a near-field microscopy application is hereinafter referred to as a sub-wavelength aperture. The confocal scanning probe employs diffraction-limited optics to achieve resolution of the order of the optical wavelength. Spatially extended images are acquired by driving the scanning probe in a raster pattern.
Effects of background beams in certain near-field microscopy systems generally are a significant source of systematic and statistical errors.
The invention features systems and methods for near-field, interferometric microscopy in which one or more phase retardation plates are positioned in the measurement and/or reference arms to reduce the contribution to the interference signal of background sources including, e.g., a beam component scattered from a near-field aperture used to couple a probe beam to a sample. The systems may operate in either reflective or transmissive modes. Furthermore, the microscopy systems using the aperture may be designed to investigate the profile of a sample, to read optical date from a sample, and/or write optical date to a sample.
In general, in one aspect, the invention features an interferometric optical microscopy system for imaging an object. The system includes: (i) a beam splitter positioned to separate an input beam into a measurement beam and a reference beam; (ii) a measurement beam source array positioned to receive the measurement beam, the measurement beam source array including a mask having an array of measurement apertures each configured to radiate a portion of the measurement beam to the object, the object interacting with the radiated measurement beam portions to direct signal radiation back through the apertures to define a measurement return beam; (iii) a reference beam source array positioned to receive the reference beam, the reference beam source array including an array of elements each configured to radiate a portion of the reference beam, the radiated reference beam portions defining a reference return beam; (iv) a multi-element photo-detector; (v) imaging optics positioned to direct the measurement and reference return beams to the photo-detector and configured to produce overlapping conjugate images of the array of reference elements and the array of measurement apertures on the photo-detector, wherein the conjugate image for each measurement aperture overlaps with the conjugate image of a corresponding reference element to produce an optical interference signal indicative of a particular region of the object; and (vi) at least one phase mask positioned to contact the return measurement beam and the return reference beam, wherein the at least one phase mask causes the conjugate image for each reference element and each measurement aperture to have an asymmetric profile along at least a first dimension.
Embodiments of the microscopy system may include any of the following features.
The at least one phase mask may include a first phase mask and a second phase mask, wherein the first phase mask is positioned to contact the return measurement beam and not the return reference beam, and wherein the second phase mask is positioned to contact the return reference beam and not the return measurement beam. For example, the first phase mask may be positioned in a pupil plane of the imaging optics for the return measurement beam and the second phase mask may be positioned in a pupil plane of the imaging optics for the return reference beam.
Alternatively, the at least one phase mask may include a first phase mask positioned to contact both of the return measurement beam and the return reference beam. For example, that first phase mask is positioned in a pupil plane of the imaging optics for both the return measurement beam and the return reference beam.
The at least one phase mask may divide the transverse profile of the return measurement beam and the return reference beam into multiple sections along the first dimension and imparts a relative phase shift equal to xcfx80+2xcfx80n, where is n is an integer, to half of the multiple sections.
The phase mask may further cause the conjugate image for each reference element and each measurement aperture to have an asymmetric profile along a second dimension orthogonal to the first dimension. For example, the at least one phase mask may divide the transverse profile of the return measurement beam and the return reference beam along the first dimension into multiple sections and impart a relative phase shift equal to xcfx80+2xcfx80n1, where is n1 is an integer, to half of the multiple sections corresponding to the first dimension, and the at least one phase mask may further divide the transverse profile of the return measurement beam and the return reference beam along a second dimension orthogonal to the first dimension into multiple sections and impart a relative phase shift equal to xcfx80+2xcfx80n2, where is n2 is an integer, to half of the multiple sections corresponding to the second dimension.
The system may further include a second, at least one phase mask positioned to contact the return measurement beam and the return reference beams, wherein the second, at least one phase mask causes the conjugate image for each reference element and each measurement aperture to have an asymmetric profile along a second dimension orthogonal to the first dimension.
The system may further include a pinhole array positioned adjacent the photodetector, wherein each pinhole is aligned with a separate set of one or more detector elements, and wherein the imaging system causes the conjugate image for each measurement aperture to align with a corresponding pinhole of the pinhole array.
The mask in the measurement beam source array may further include an array of measurement scattering elements, wherein each measurement scattering element is adjacent a corresponding one of the measurement apertures and has transverse dimensions comparable to the corresponding measurement aperture. Each measurement scattering element scatters a portion of the measurement beam, and the measurement return beam further includes the portions of the measurement beam scattered by the measurement scattering elements. In such cases, the imaging optics are further configured to produce a conjugate image of the array of measurement scattering elements that overlaps with the conjugate image of the array of reference elements, wherein the conjugate image for each measurement scattering element overlaps with the conjugate image of a corresponding reference element to produce an optical interference signal that provides an estimate of scattering from the adjacent measurement aperture.
For embodiments including the scattering elements, the system may further include a pinhole array positioned adjacent the photo-detector, wherein each pinhole is aligned with a separate set of one or more detector elements, and wherein the imaging system causes the conjugate image for each measurement aperture and each measurement scattering element to align with a corresponding pinhole of the pinhole array.
Each reference element may include a reflective element.
Each reference element may include a transmissive aperture.
In general, in another aspect, the invention features an interferometric optical microscopy system for imaging an object, the system including: (i) a beam splitter positioned to separate an input beam into a measurement beam and a reference beam; (ii) a measurement beam source array positioned to receive the measurement beam, the measurement beam source array including a mask having an array of source apertures each configured to radiate a portion of the measurement beam to the object; (iii) a measurement beam detector array including a mask having an array of measurement apertures, wherein the object interacts with the radiated measurement beam portions and directs the resulting signal radiation through the measurement apertures to define a measurement return beam; (iv) a reference beam source array positioned to receive the reference beam, the reference beam source array including an array of elements each configured to radiate a portion of the reference beam, the radiated reference beam portions defining a reference return beam; (v) a multi-element photo-detector; (vi) imaging optics positioned to direct the measurement and reference return beams to the photo-detector and configured to produce overlapping conjugate images of the array of reference elements and the array of measurement apertures on the photo-detector, wherein the conjugate image for each measurement aperture overlaps with the conjugate image of a corresponding reference element to produce an optical interference signal indicative of a particular region of the object; and (vii) at least one phase mask positioned to contact the return measurement beam and the return reference beams, wherein the at least one phase mask causes the conjugate image for each reference element and each measurement aperture to have an asymmetric profile along at least a first dimension.
Embodiments of the microscopy system may include any of the following features.
The at least one phase mask may include a first phase mask and a second phase mask, wherein the first phase mask is positioned to contact the return measurement beam and not the return reference beam, and wherein the second phase mask is positioned to contact the return reference beam and not the return measurement beam. For example, the first phase mask may be positioned in a pupil plane of the imaging optics for the return measurement beam and the second phase mask may be positioned in a pupil plane of the imaging optics for the return reference beam.
Alternatively, the at least one phase mask may include a first phase mask positioned to contact both of the return measurement beam and the return reference beam. For example, that first phase mask is positioned in a pupil plane of the imaging optics for both the return measurement beam and the return reference beam.
The at least one phase mask may divide the transverse profile of the return measurement beam and the return reference beam into multiple sections along the first dimension and imparts a relative phase shift equal to xcfx80+2xcfx80n, where is n is an integer, to half of the multiple sections.
The phase mask may further cause the conjugate image for each reference element and each measurement aperture to have an asymmetric profile along a second dimension orthogonal to the first dimension. For example, the at least one phase mask may divide the transverse profile of the return measurement beam and the return reference beam along the first dimension into multiple sections and impart a relative phase shift equal to xcfx80+2xcfx80n1, where is n1 is an integer, to half of the multiple sections corresponding to the first dimension, and the at least one phase mask may further divide the transverse profile of the return measurement beam and the return reference beam along a second dimension orthogonal to the first dimension into multiple sections and impart a relative phase shift equal to xcfx80+2xcfx80n2, where is n2 is an integer, to half of the multiple sections corresponding to the second dimension.
The system may further include a second, at least one phase mask positioned to contact the return measurement beam and the return reference beams, wherein the second, at least one phase mask causes the conjugate image for each reference element and each measurement aperture to have an asymmetric profile along a second dimension orthogonal to the first dimension.
The system may further include a pinhole array positioned adjacent the photodetector, wherein each pinhole is aligned with a separate set of one or more detector elements, and wherein the imaging system causes the conjugate image for each measurement aperture to align with a corresponding pinhole of the pinhole array.
Each reference element may include a reflective element.
Each reference element may include a transmissive aperture.
In general, in another aspect, the invention features an interferometric, microscopy method for imaging an object, the method including: (i) separating an input beam into a measurement beam and a reference beam; (ii) directing the measurement beam to an array of measurement apertures to cause a portion of the measurement beam to couple through each of the measurement apertures and radiate the object; (iii) receiving signal radiation from the object back through the apertures in response to the radiated measurement beam portions, wherein the signal radiation defines a measurement return beam; (iv) directing the reference beam to array of reference elements to cause each configured to radiate a portion of the reference beam, the radiated reference beam portions defining a reference return beam; (v) imaging the measurement and reference return beams onto a photo-detector to produce overlapping conjugate images of the array of reference elements and the array of measurement apertures on the photo-detector, wherein the conjugate image for each measurement aperture overlaps with the conjugate image of a corresponding reference element to produce an optical interference signal indicative of a particular region of the object; and (vi) imparting a phase pattern onto each of the return measurement beam and the return reference beams that causes the conjugate image for each reference element and each measurement aperture to have an asymmetric profile along at least a first dimension.
In general, in another aspect, the invention features an interferometric, microscopy method for imaging an object, the method including: (i) separating an input beam into a measurement beam and a reference beam; (ii) directing the measurement beam to an array of source apertures to cause a portion of the measurement beam to couple through each of the measurement apertures and radiate the object; (iii) receiving signal radiation from the object through an array of measurement apertures in response to the radiated measurement beam portions, wherein the signal radiation defines a measurement return beam; (iv) directing the reference beam to array of reference elements to cause each configured to radiate a portion of the reference beam, the radiated reference beam portions defining a reference return beam; (v) imaging the measurement and reference return beams onto a photo-detector to produce overlapping conjugate images of the array of reference elements and the array of measurement apertures on the photo-detector, wherein the conjugate image for each measurement aperture overlaps with the conjugate image of a corresponding reference element to produce an optical interference signal indicative of a particular region of the object; and (vi) imparting a phase pattern onto each of the return measurement beam and the return reference beams that causes the conjugate image for each reference element and each measurement aperture to have an asymmetric profile along at least a first dimension.
Embodiments of either of the above methods may further include features corresponding to any of the features described above in connection with the microscopy systems.
Confocal and near-field confocal, microscopy systems are also described in the following, commonly-owed provisional applications: Ser. No. 09/631,230 filed Aug. 2, 2000 by Henry A. Hill entitled xe2x80x9cScanning Interferometric Near-Field Confocal Microscopy,xe2x80x9d and the corresponding PCT Publication WO 01/09662 A2 published Feb. 8, 2001; Provisional Application Serial No. 60/221,019 filed Jul. 27, 2000 by Henry A. Hill and Kyle B. Ferrio entitled xe2x80x9cMultiple-Source Arrays For Confocal And Near-Field Microscopyxe2x80x9d and the corresponding Utility application Ser. No. 09/917,402 having the same title filed on Jul. 27, 2001; Provisional Application Serial No. 60/221,086 filed Jul. 27, 2000 by Henry A. Hill entitled xe2x80x9cControl of Position and Orientation of Sub-Wavelength Aperture Array in Near-Field Microscopyxe2x80x9d and the corresponding Utility application Ser. No. 09/917,401 having the same title filed on Jul. 27, 2001; Provisional Application Serial No. 60/221,091 filed Jul. 27, 2000 by Henry A. Hill entitled xe2x80x9cMultiple-Source Arrays with Optical Transmission Enhanced by Resonant Cavities and the corresponding Utility application Ser. No. 09/917,400 having the same title filed on Jul. 27, 2001; and Provisional Application Serial No. 60/221,295 by Henry A. Hill filed Jul. 27, 2000 entitled xe2x80x9cDifferential Interferometric Confocal Near-Field Microscopyxe2x80x9d and the corresponding Utility application Ser. No. 09/917,276 having the same title filed on Jul. 27, 2001; the contents of each of the preceding applications being incorporated herein by reference. Aspects and features disclosed in the preceding provisional applications may be incorporated into the embodiments described in the present application.
Embodiments of the invention may include any of the following advantages.
One advantage is a tomographic complex amplitude imaging technique that conveniently reduces or eliminates the statistical error effects of light from out-of-focus image points.
Another advantage is an improved technique for tomographic complex amplitude imaging wherein systematic error effects of out-of-focus light images are greatly reduced or eliminated.
Another advantage is a tomographic complex amplitude imaging technique which allows substantially simultaneous imaging of an object at multiple image points.
Another advantage is a convenient technique for tomographic complex amplitude imaging in with the means to obtain a signal-to-noise ratio for the images that is achievable with an interferometric system.
Another advantage is a tomographic complex amplitude imaging system and technique which avoids the computation difficulties of solving nonlinear differential equations.
Another advantage is an interferometric profiler based on interferometry of near-field beams.
Another advantage is an interferometric profiler based on interferometric confocal microscopy of near-field beams.
Another advantage is a scanning interferometric near-field confocal microscope operating in a continuous scan mode with a pulsed input optical beam.
Another advantage is a scanning interferometric near-field confocal microscope with enhanced signal-to-noise ratios for measured complex amplitudes of scattered/reflected near-field beams by an object material.
Another advantage is a simpler inversion type of calculation for properties of object being profiled and/or imaged.
Another advantage is that the directions of propagation of components of a near-field probe beam at a given volume section of an object being profiled/imaged are substantially the same for a given measured amplitude and phase of a reflected/scattered near-field probe beam from the volume section wherein the dimensions of the volume section are much less than the dimensions of the source of the near-field probe beam.
Another advantage is that amplitudes and phases of reflected/scattered near-field probe beams by an object being profiled/imaged are measured for substantially low order electric and magnetic multipole near-field sources wherein the dominant multipole sources are three different near-field probe beam sources comprising an electric dipole and two different orthogonal orientations of a magnetic dipole.
Another advantage is that effects of an interference term between a back-ground beam produced by scattering and/or reflections of an antecedent beam of a beam generating a near-field probe beam and a reflected/scattered portion of the near-field probe beam by an object being profiled/imaged are compensated.
Another advantage is that effects of an interference term between a back-ground beam produced by scattering and/or reflections of an antecedent beam of a beam generating a near-field probe beam and a reference beam are compensated.
Another advantage is that statistical errors in measured amplitudes and phases of reflected/scattered near-field probe beams by an object being profiled/imaged can be substantially the same as statistical errors based on Poisson statistics of the reflected/scattered near-field probe beams, i.e. not significantly degraded by the presence of background signals.
Another advantage is that a wavelength dependence of amplitudes and phases of reflected/scattered near-field probe beams for near-field probe beams generated by different near-field probe beam sources substantially comprising an electric dipole and two different orthogonal orientations of a magnetic dipole is measured.
Another advantage is that a radial dependence of amplitudes and phases of reflected/scattered near-field probe beams for near-field probe beams generated by different near-field probe beam sources substantially comprising an electric dipole and two different orthogonal orientations of a magnetic dipole is measured.
Another advantage is that an angular dependence of amplitudes and phases of reflected/scattered near-field probe beams for near-field probe beams generated by different near-field probe beam sources substantially comprising an electric dipole and two different orthogonal orientations of a magnetic dipole is measured.
Another advantage is that a resolution of the invention is only weakly dependent on optical resolution of an optical system imaging spatially filtered reflected/scattered near-field probe beams from an object being profiled/imaged onto pixels of a detector wherein the spatially filtering is achieved by an array of wavelength and/or sub-wavelength apertures.
Another advantage is that a source of a near-field probe beam may be a pulsed source.
Another advantage is that scanning of an object being profiled/imaged may be implemented in a xe2x80x9cstep and repeatxe2x80x9d mode or in a continuous scan mode.
Another advantage is that measured interference terms between spatially filtered reflected/scattered near-field probe beams and reference beams can be obtained substantially simultaneously for a one-dimensional or a two-dimensional array of locations on an object being profiled/imaged.
Another advantage is that a rotation in a plane of polarization of a reflected/scattered near-field probe beam by an object being profiled/imaged due to a given state of magnetization of the object at a point in or on the object is measured.
Another advantage is that statistical errors in the measured values of intensity for each point in an one-dimensional or two-dimensional arrays of measured intensity values are the same as the statistical error acquired in a measured intensity value for a single pinhole interferometric confocal near-field microscope.
Another advantage is that a rotation in a state of polarization of a reflected/scattered near-field probe beam by an object being profiled/imaged, due for example to a state of magnetization or a changes in a state of magnetization of the object at a point in or on the object is measured.
Another advantage is that the invention may be used to write to an optical data storage medium such as a magneto-optical material.
Another advantage is that the invention profiles a surface and internal layers near the surface of an object being profiled/imaged without contacting the object.
Another advantage is that a wavelength of a source of a near-field probe beam may be in the ultraviolet, visible, or the infrared. For example, the source may comprise two or more different wavelengths.
Another advantage is that either optical heterodyne or homodyne techniques may be used to measure amplitudes and phases of interference terms between a reference beam and a reflected/scattered near-field probe beam from an object being profiled/imaged.
Another advantage is that an effective spatial resolution of the invention may be improved over the resolution obtained with non-interferometric near-field microscopy by the combination of information obtained from measured amplitudes and phases of reflected/scattered near-field probe beams for different types of near-field probe beam sources, i.e. substantially an electric dipole and two different orientations of a magnetic dipole.
Another advantage is that a complex value of an index of refraction for an object being profiled/imaged can be determined from measured arrays of amplitudes and phases of reflected/scattered near-field probe beams produced by reflection/scattering by the object, wherein the dimensionality of the arrays may comprise one or two dimensions corresponding to one and two dimensions of space, a dimension for the spatial separation of the source of the near-field probe beam and the object, a dimension for each of wavelength of components of the near-field probe beam source, and a dimension for the multipole characterization of the source of the near-field probe beam.
Another advantage is that multiple layers of optical data stored on and/or in an optical storage medium can be read substantially simultaneously.
Another advantage is that a source of near-field probe beams can be sub-wavelength apertures in a sub-wavelength thick conducting layer.
Another advantage is that a source of a near-field probe beam(s) may comprise wavelength and sub-wavelength Fresnel zone plate(s).
Another advantage is that microlenses may be added to a source(s) of near-field probe beam(s) to alter properties of the near-field probe beam(s) at an object being profiled/imaged.
Another advantage is that gratings may be added to an array of wavelength or sub-wavelength apertures operating as spatial filters of reflected/scattered or transmitted near-field probe beam(s) to alter properties of the reflected/scattered or transmitted near-field probe beam(s).
Another advantage is that an optical system imaging onto pixels of a detector spatially filtered reflected/scattered near-field probe beam from an object being profiled/imaged has a depth resolution of a pinhole confocal microscope.
Another advantage is that microlenses may be added to a source(s) of near-field probe beam(s) to alter properties of the near-field probe beam(s) at an object being profiled/imaged.
Another advantage is that gratings may be added to a source(s) of near-field probe beam(s) to alter properties of the near-field probe beam(s) at an object being profiled/imaged.
Another advantage is that gratings may be added to an array of wavelength/sub-wavelength apertures operating as spatial filters of reflected/scattered or transmitted near-field probe beam(s) to alter properties of the reflected/scattered or transmitted near-field probe beam(s).
Another advantage is that a change in temperature of a site in or on an object being profiled/imaged can be detected as a change in the complex value of the index of refraction of the object at the site.
Another advantage is that an angular distribution of a reflected/scattered near-field probe beam from an object being profiled/imaged is measured to obtain information about a multipole nature of the reflected/scattered near-field probe beam.
Another advantage is that an angular distribution of a reflected/scattered near-field probe beam from a wavelength or sub-wavelength size structure in/or on an object being imaged or profiled is measured.
Another advantage is that if it is necessary to correct for out-of-focus images which are already greatly reduced in the apparatus, the computer processing required to achieve a given level of correction with the apparatus is significantly reduced compared to the computer processing required in prior art scanning single-pinhole and scanning slit confocal and scanning single-pinhole and scanning slit confocal interferometric microscopy.
Other features, aspects, and advantages follow.